Biogas is comprised of two major compounds (i.e., CH4 and CO2) derived from fermentation of organic wastes. Therefore, biogas can be used as a source for the generation of syngas (H2 and CO: through ...dry reforming of methane). Given that the dominant fraction of biogas is consumed as a feedstock for lower-end products, such as heat and power, dry reforming can be used as an effective option for the valorization of biogas. In this review, we offer up-to-date knowledge on the development of biogas dry reforming in the context of the effects of the composition of the biogas, reaction conditions, and impurities in the biogas. Theoretical estimations of biogas compositions were made along with the compositional matrix of organic substrates. The thermodynamic calculations of dry reforming were also described with other side reactions. In conclusion, the challenges and the potential future directions of this research field were given to help open up new paths toward hybrid biological/chemical processes for H2 production.
•Theoretical estimations and practical results of biogas compositions were discussed.•Up-to-date knowledge on biogas reforming and technical challenges were addressed.•A hybrid platform for H2 production from organic wastes was discussed.
Understanding the structure–catalytic activity relationship is crucial for developing new catalysts with desired performance. In this contribution, we report the performance of In2O3 with different ...crystal phases in the reverse water gas shift (RWGS) reaction, where we observe changing activity induced by a phase transition under reaction conditions. Cubic In2O3 (c-In2O3) exhibits a higher RWGS rate than the hexagonal phase (h-In2O3) at temperatures below 350 °C because of its (1) enhanced dissociative adsorption of H2, (2) facile formation of the oxygen vacancies, and (3) enhanced ability to adsorb and activate CO2 on the oxygen vacancies, as suggested both experimentally and computationally. Density functional theory results indicate that the surface oxygen arrangement on the cubic polymorph is key to rapid H2 adsorption, which facilitates oxygen vacancy formation and subsequent CO2 adsorption to yield high RWGS reactivity. At 450 °C and above, the activity of h-In2O3 increases gradually with time on stream, which is caused by a phase transition from h-In2O3 to c-In2O3. In situ X-ray diffraction experiments show that h-In2O3 is first reduced by H2 and subsequently oxidized by CO2 to c-In2O3. These findings highlight the importance of the crystal phase in the catalytic RWGS reaction and provide a new dimension for understanding/designing RWGS catalysts.
Hierarchically combining semi-empirical methods and first-principles calculations we gain a novel and noteworthy picture of the molecular-level mechanisms that govern the water–gas-shift (WGS) and ...reverse water–gas-shift (r-WGS) reactions on Rh catalysts. Central to this picture is that the WGS and r-WGS follow two different dominant reaction mechanisms: WGS proceeds according to a carboxyl (COOH) mechanism, whereas r-WGS proceeds according to a redox (CO2→CO+O) mechanism.
The obtained results furthermore underscore the danger of common first-principles analyses that focus on a priori selected dominant paths. Not restricted to such bias, our herein proposed hierarchical approach thus constitutes a promising avenue to properly transport and incorporate the ab initio predictive-quality to a new level of system complexity.
The reverse water‐gas shift reaction (RWGS) has been regarded as a promising approach for fighting climate change caused by the excessive emission of the greenhouse gas CO2, but it still suffers from ...relatively poor low‐temperature reactivity. Herein, a high‐performance RWGS catalyst composed of ultrasmall Pt clusters (1.38 nm on average) anchored by La2O2CO3 support (PtNC/LOC), which possesses a record CO production rate of 2678 molCO molPt−1 h−1 with nearly 100% CO selectivity at 300 °C, is reported. The specific activity is nearly 1.5 and 7.9 fold higher than that of Pt single atoms and nanoparticles on La2O2CO3, respectively. More importantly, over 87.7% of the initial activity of PtNC/LOC remains after 80 h constant operation at 380 °C, firmly verifying the structural robustness. LOC support is essential, because of not only the moderate basicity that can boost the reaction efficiency but also the strong interaction with Pt species that can stabilize the ultrasmall particle size. Further investigations also point out the key role of the Pt clusters. The two key steps, CO2 adsorption, and CO desorption, individually prefer electron‐rich and electron‐deficient Pt species. Ultrasmall Pt clusters successfully integrate the unique surface states of single atoms and nanoparticles, thus achieving excellent low‐temperature RWGS performance.
A series of La2O2CO3‐based catalysts are developed and used as advanced catalysts for the reverse water‐gas shift reaction. Compared with their single atom and nanoparticle counterparts, the La2O2CO3‐supported Pt clusters achieve a better balance between CO2 adsorption and CO desorption, thus leading to an accelerated reaction rate.
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•This review introduces the WGS reaction and catalyst of diverse types of syngas.•Natural gas-, biomass-, waste-, and coal-derived syngas are categorized.•Customized reaction ...conditions/catalysts are compared based on syngas type.•Some similarities were identified through the comparison of syngas characteristics.•CO concentration and the presence of H2S are the main consideration factors.
The conventional hydrogen production process from natural gas includes a water–gas shift reaction (WGSR) as a core step to remove carbon monoxide and produce additional hydrogen. The WGSR can be further applied to the upcycling of other types of synthesis gases, such as biomass, municipal solid waste, and coal-derived synthesis gas. We have focused on the reaction conditions and catalysts for the WGSR dealing with diverse types of feed gases for the last 10 years to understand the development progress. Based on the categorization (by the type of feed gas), the tested catalysts, capacity, temperature, feed gas composition, steam-to-carbon ratio, and the performance of the catalyst are carefully compared. This review provides insight into the current research trends and perspectives for target-oriented WGSR in each type of feed gas, which can give clues for customization.
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•WGS reaction on CeO2 (111)-supported Pt cluster follow both redox and associative carboxyl with redox regeneration mechanisms.•High activity of Pt/CeO2 interface sites originates ...from a significantly enhanced water activation and dissociation at interfacial oxygen vacancies.•First principles-based microkinetic modeling analysis provides insights on the unique activity of Pt/CeO2 interface.
The mechanism of water–gas shift reaction at the three-phase boundary of Pt/CeO2 catalysts has been investigated using density functional theory and microkinetic modeling to better understand the importance of metal–oxide interface sites in heterogeneous catalysis. Analysis of a microkinetic model based on parameters obtained from first principles suggests that both the “Redox pathway” and the “Associative carboxyl pathway with redox regeneration” could operate on Pt/CeO2 catalysts. Although (1) only few interfacial Pt atoms are found to be catalytically active at low temperatures due to strong adsorption of CO and (2) interfacial O–H bond breakage is difficult due to the high reducibility of ceria, interface sites are 2–3 orders of magnitude more active than Pt (111) and stepped Pt surface sites and therefore effectively determine the overall activity of Pt/CeO2. The high activity of Pt/CeO2 interface sites originates from a significantly enhanced water activation and dissociation at interfacial oxygen vacancies.
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•Zn-based MOF-74 was assembled and controlled as nanoshuttles morphology.•Core-shell Au@Pd functional nanoparticles were encapsulated into MOF-74.•Pt nanoparticles were deposited on ...the surface of MOF-74 and Au@Pd@MOF-74.•The composite catalysts exhibit selective catalytic effectiveness for CO2 conversion.
The Zn-based metal-organic framework (MOF) catalysts MOF-74, Au@Pd@MOF-74, Pt/MOF-74, and Pt/Au@Pd@MOF-74, were assembled. Core-shell Au@Pd, in which Au nanoparticles (NPs) serve as the core for the epitaxial growth of Pd shells, was encapsulated into MOF-74 nanoshuttles to control the MOF-74 morphology and impart NP functionality. Pt NPs were loaded onto the MOF-74 surface. Pt/MOF-74 catalyzed CO2 conversion at 99.7% CO selectivity in the reverse water-gas shift (RWGS) reaction, and Au@Pd@MOF-74 photocatalyzed CO2 with 100% CO selectivity. Importantly, the catalyst Pt/Au@Pd@MOF-74, not only catalyzed the conversion of CO2 to CO in RWGS with 99.6% selectivity, but also photocatalyzed CO2 to CH4 at low concentration (800 ppm).
It is crucial to probe the effect of the alkali metal additives on the forward and reverse water–gas shift reaction (WGSR and RWGSR). Density functional theory (DFT) calculations were performed to ...investigate the reaction mechanism and activity of WGSR on clean and K-modified Cu(111) and Cu(110) surfaces. The calculation results indicate that the K adatom greatly stabilizes the adsorption of all oxygenate intermediates but rarely affects the binding strength of other species (e.g., H, H2). More importantly, it is found that the K adatom can improve the reactivity of WGSR and RWGSR on Cu catalysts by favoring their rate-limiting steps of H2O and CO2 dissociation and reducing the apparent activation energy of the whole reaction. On the basis of energetic and electronic analysis, the promoting effect of K on H2O and CO2 dissociation can be attributed to the direct bonding forming between the atomic K and O of oxygenate species at the TSs, and more, stronger K–O bonding on Cu(111) than on Cu(110) contributes to the stronger promoting effect of K on WGSR and RWGSR on the (111) surface. The effects of other alkali metals (Na, Rb, and Cs) on H2O and CO2 dissociation were also explored, and the different promoting effects are due to different electronegativities of alkali metals, which induce different work function changes and surface dipole moments; the lower the work function or the higher the surface dipole moment, the stronger the promoting effect of alkali metal.
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•Recent advances in catalyst design for the RWGS reaction are studied.•CO adsorption energy in transition metals may affect their CO2 hydrogenation product selectivity.•Different ...reaction mechanisms for the RWGS reaction are proposed.•Electron density, catalyst structure, and the presence of hydroxyl groups are vital for catalyst design.
Synthesis gas production through the catalytic reverse water-gas shift (RWGS) reaction is an attractive option for the conversion of CO2 to fuels. Many metal-based catalysts have been introduced for this reaction in order to provide high activity, CO selectivity, and stability. Recently, progress has been made in catalyst design and understanding of the reaction mechanism, which has shed light on the characteristics of the catalysts needed for this reaction. Accordingly, new noble and non-noble metal-based catalysts with remarkable performance have been introduced for this reaction. However, there is still much room for catalyst improvement specifically in regard to catalyst stability at the high temperatures required for this reaction. There are also controversial arguments regarding the active sites of the reaction. This review highlights the recent progress in catalyst design and understanding of the reaction mechanism for the RWGS reaction and derives proposals for further improvements of the process.
The ever growing increase of CO2 concentration in the atmosphere is one of the main causes of global warming. Thus, CO2 activation and conversion toward valuable added compounds is a major scientific ...challenge. A new set of Au/δ-MoC and Cu/δ-MoC catalysts exhibits high activity, selectivity, and stability for the reduction of CO2 to CO with some subsequent selective hydrogenation toward methanol. Sophisticated experiments under controlled conditions and calculations based on density functional theory have been used to study the unique behavior of these systems. A detailed comparison of the behavior of Au/β-Mo2C and Au/δ-MoC catalysts provides evidence of the impact of the metal/carbon ratio in the carbide on the performance of the catalysts. The present results show that this ratio governs the chemical behavior of the carbide and the properties of the admetal, up to the point of being able to switch the rate and mechanism of the process for CO2 conversion. A control of the metal/carbon ratio paves the road for an efficient reutilization of this environmental harmful greenhouse gas.
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